Lets do a little Math... A cable that moves 4GW of electricity 450 km is say +-1 million volts and 2000 amps.
Assuming we want no more than 3% losses in the cable at full load, then each conductor needs to be 40mm diameter aluminium, at a total material cost of $3.6M
To insulate a 1 million volt cable, we need 100mm of PVC - total cost $60M.
And we'll obviously need a few mm of steel + more PVC on the outside for protection from the environment.
And now add in the manufacturing cost, and the cost to get it into place...